1. Field of the Invention
The present invention relates to an electroluminescent lamp, and more particularly to an electroluminescent lamp having features such as noise prevention and electromagnetic shield, and which is suitable for use in backlighting of a liquid-crystal display.
2. Description of the Related Art
In recent years, with the rapid increase in use of such electronic equipment as cellular phones, PHS (Personal Handyphone System microcellular phones), radio pagers, digital watches, and portable personal computers, there has been an increase in the demand for thin surface illumination type electroluminescent lamps for use in backlighting of liquid-crystal displays.
When an electroluminescent lamp is caused to emit light by the application thereto of a high AC voltage, noise is generated, either from the electroluminescent lamp itself, or by an electrostatic coupling between the electroluminescent lamp and the liquid-crystal display.
Additionally, noise that is generated from inverters or other circuitry used to drive the electroluminescent lamp passes through the electroluminescent lamp and has an adverse affect on the liquid crystal display or electronic equipment.
For this reason, three-electrode electroluminescent lamps that with reduced noise and electromagnetic shielding have been developed. Such an electroluminescent lamp 30 is shown in an expanded cross-sectional view in FIG. 4, this having been described in the Japanese Unexamined Utility Model Publication (KOKAI) No. 3-37795.
As shown in FIG. 4, on the reverse, non-light-emitting side of an electroluminescent lamp 40 a third electrode sheet 50 is attached in intimate contact, using an adhesive.
The usual construction of the electroluminescent lamp 40 has such a configuration in that the lamp 40 includes an electroluminescent element 45, comprising a lamination of a reverse-side electrode 41 of aluminum foil or the like, a reflective insulation layer 42 made of barium titanate powder dispersed in a resin binder, a light-emitting layer 43 of a zinc sulfide phosphor dispersed in a resin binder, a transparent electrode 44 made of ITO or the like formed onto a transparent film, and the electroluminescent element 45, is sealed from the top and bottom thereof by external moisture prevention sealing films 48 and 49, with remaining lead electrodes 46 and 47 in extended condition from the transparent electrode 44 and the reverse-side electrode 41, respectively.
The third electrode sheet 50 is formed by sealing and insulating a metal foil 51 of aluminum or the like from the top and bottom by resin films 52 and 53, and making connection to the end thereof by a lead 54.
By electrically connecting the lead 54 of the third electrode sheet 50 to the lead electrode 46 of the transparent electrode 44 of the electroluminescent lamp 40, the transparent electrode 44 that is positioned at both sides of the reverse-side electrode 41 and the third electrode 51 are placed at the same potential, thereby preventing noise when AC drive is done.
If grounding is done, it is possible to achieve electromagnetic shielding.
Therefore, if this three-electrode electroluminescent lamp 30 is used as a backlight for a liquid crystal, because the high AC voltage that is applied to the EL element is shielded, not only is electromagnetic noise reduced, but also the electrostatic coupling with respect to the liquid crystal is blocked, this reducing the mutual interaction therebetween, so as to reduce noise.
In recent years, with rapid advancements in making more compact and lightweight electronic equipment having liquid-crystal displays, there has been an increase in the demand for thin electroluminescent lamps with reduced noise and electromagnetic shielding for use in backlighting for liquid-crystal displays.
However, with a electroluminescent lamp 30 having a three-electrode construction as in the past, because the third electrode sheet 50 construction is that of a metal foil 51 of aluminum or the like sealed by the resin sheets 52 and 53 from the top and bottom, to prevent pressure and the like applied during the sealing process from causing cracking and deformation of the metal foil 51, it is not possible to make the thickness of the metal foil 51 thin, this being required to be at least 35 .mu.m.
For this reason, the overall thickness of the third electrode sheet as it is sandwiched between the resin sheets 52 and 53 was approximately 0.1 mm.
Additionally, the existence of the external moisture prevention sealing films 48 and 49 that seal the electroluminescent element 45 also hindered the achievement of a thin electroluminescent lamp, and when this is included the thickness of the electroluminescent lamp 40 became approximately 0.8 mm.
Therefore, the overall thickness of an electroluminescent lamp 30, formed by the overlaying of the electroluminescent lamp 40 and the third electrode sheet 50, was approximately 0.9 mm, this being extremely thick.
Accordingly, it is an object of the present invention to provide an electroluminescent lamp with noise prevention and electromagnetic shielding that is suitable for use as a backlight for a liquid-crystal display in, for example, portable electronic equipment.